In an attempt to reduce atmospheric pollution, many countries are imposing increasingly stringent limits on the composition of the exhaust gases produced by internal combustion engines and released into the atmosphere. The primary harmful substances from internal combustion engines include hydrocarbons, carbon monoxide, nitrogen oxides (NOx) and particulate matter. Heretofore, many methods have been proposed in an attempt to reduce or minimize the quantity of such substances present in the exhaust gases emitted into the environment.
The use of honeycomb structures as filters for removing particulates (e.g., soot) from engine exhaust gases, and as substrates for supporting catalytic materials for purifying engine exhaust gases is known. A particulate filter body may be, for example, a honeycomb article having a matrix of intersecting thin, porous walls that extend across and between its two opposing open end faces and form a large number of adjoining hollow passages, or cells, which also extend between and are open at the end faces. To form a filter, a first subset of cells is closed at one end face, and the remaining cells are closed at the other end face. A contaminated gas is brought under pressure to one face (the “inlet face”) and enters the filter body via the cells that are open at the inlet face (the “inlet cells”). Because the inlet cells are sealed at the remaining end face (the “outlet face”) of the body, the contaminated gas is forced through the thin, porous walls into adjoining cells that are sealed at the inlet face and open at the opposing outlet face of the filter body (the “outlet cells”). The solid particulate contaminants in the exhaust gas (such as soot), which are too large to pass through the porous openings in the walls, are left behind, and cleaned exhaust gas exits the outlet face of the filter body through the outlet cells.
A substrate for supporting catalytic materials may similarly be a honeycomb structure having a matrix of intersecting walls that extend across and between its two opposing open end faces and form a large number of adjoining hollow passages, or cells, which also extend between and are open at the end faces. The walls are coated with a catalytic material selected to reduce the amount of carbon monoxide (CO), nitrogen oxides (NOx), and/or unburned hydrocarbons (HC) in the exhaust gas as the exhaust gas passes through the cells. These honeycomb structures (i.e., filters and substrates) may have transverse cross-sectional cellular densities of approximately 1/10 to 100 cells or more per square centimeter.
Such honeycomb structures are typically formed by an extrusion process where a material is extruded in a green (uncured) body before the green body is fired to form the final ceramic material of the honeycomb structure. The extruded green bodies can be any size or shape and have relatively low mechanical strength. As used herein, ceramic greenware, or more briefly greenware, refers to bodies comprised of ceramic-forming components that, upon firing at high temperature, form ceramic bodies. The greenware may include ceramic-forming precursor components, ceramic components, and mixtures of various ceramic-forming components and ceramic components. The various components can be mixed together with a liquid vehicle such as, for example, water or glycol. Immediately after extrusion, the greenware possesses some given liquid vehicle content, such as a water or glycol content, at least some of which must be removed, i.e., the greenware must be dried, prior to firing at high temperature.
The drying process must be carried out in a manner that does not cause defects the greenware, such as shape change, cracks, fissures, and the like. Such defects tend to occur when the greenware is overheated during the drying process.